It is known in the art to form guidewires for placement in the human anatomy. Many guidewires are fabricated from a superelastic Nitinol (nickel titanium) alloy. In some known methods of guidewire fabrication and assembly, the distal end of the nitinol core is “pretinned” with a solder alloy to enable subsequent assembly by soldering a coil onto the distal end.
Such known methods may include a pretinning process which involves a heated crucible containing several pounds of molten solder and a cover layer of molten flux. The molten solder may be a mixture of tin and gold, and as a consequence the value of a solder station known in the art may be between $40,000 to $50,000. The flux may be a eutectic mixture of sodium hydroxide and potassium hydroxide. The distal end of each core wire is plunged or dipped into the crucible multiple times, resulting in a pretinned section approximately 5 cm long that is readily solderable at any point along that length using the type of flux typically used for stainless steel.
In some guidewire designs, the entire nitinol core wire is in the superelastic condition, having been previously cold worked via wire drawing then heat treated at about 500° C. to straighten the drawn wire and impart superelasticity. Because superelastic nitinol has already been heat treated at a higher temperature than will be encountered during the pretinning procedure, this pretinning method may affect straightness of the wire if the wire is bent while exposed to pretinning temperatures. That is, the bend will become permanent. (However, it is a characteristic of superelastic NiTi that has been previously cold worked prior to heat treatment, that its straightness will not be impacted where it is held straight while being heated in a pretinning process)
However, in other designs, the tip of the guidewire core may be treated, by various different means including by cold working, to remove the superelastic properties. This leaves the tip capable of being deformed permanently by the user. A design and method following such a process is disclosed in application Ser. No. 14/042,321 which is co-owned by the applicant company. It is incorporated herein in full, by reference. Thus, where the user requires a J tip, a micro-J tip, or other desired shape to facilitate threading the guidewire in a complex anatomy, such shape may be imparted to the tip manually before insertion into the patient. In the case of such guidewire core, the method of pretinning described above, wherein the distal end of each core wire is plunged into the crucible multiple times up to a depth of about 5 cm, causes significant distortion to occur in the cold worked section.
As further background, the need for pretinning nitinol core wires arises before they are soldered. Soldering fluxes that enable solder alloys to wet nitinol are commercially available so, in theory, pretinning before assembly should be unnecessary. However, certain alloys that are frequently used for their other beneficial characteristics, (for example, Indium Alloys' Flux #2 and Flux #3) volatilize when heated and leave considerable residue that can tenaciously mask areas where solder is intended to flow. Further, such residue can be challenging to remove, particularly when trapped within coils or other features having little clearance and poor line-of-sight inspectability.
In this respect, non-removed residue is highly undesirable because it can cause subsequent corrosion of the solder or substrate materials. For this reason, it is advantageous to pre-tin and completely remove all pre-tinning flux residue while it is exposed and readily visible, and then use an easier-to-remove flux when subsequently soldering the pre-tinnned nitinol core wires into assemblies. For example, fluxes used for soldering stainless steels or precious metals are aqueous solutions containing zinc chloride and hydrochloric acid, and their residue does not exhibit a masking effect and is far easier to remove from coils and other low-clearance features.
Thus, there is in the art, a need for a method for pretinning guidewire cores made from superelastic alloys, which avoids the above problem. The present invention addresses these and other needs.